Swimming pools, when unattended, present a considerable hazard to small children and other non-swimmers, and as a consequence, various means have been devised to deter children and others from approaching such pools. In addition to the use of physical barriers, different types of sensing systems have been designed for activating an alarm should a person enter the pool area or jump or fall into an unattended pool. For example, several float operated sensors have been designed for activating an alarm in response to wave motion of the water. One problem with these prior art sensors has been the fact that they can be completely removed from the water without actuating the alarm, thereby defeating the purpose of the system. Moreover, they have been difficult to adjust and maintain and as a consequence, they have not been used to any substantial extent.